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 Remember, most sensor's resistance goes up as temperature goes down. A
temperature increase results in a resistance decrease. If the ohmmeter shows a
direct short (zero ohms), or an open circuit (infinite ohms), check the sensor wiring
as well as the sensor itself.
Check sensor wiring for staples shorting out the wires, loose or corroded
connections and outright breaks.
Make sure sensor wiring is located away from line voltage wiring and motors.
Maintain at least 1 foot from 120V wiring and at least 2 feet from 240V. It is very
easy for a small electrical current to be induced in the low voltage sensor wires.
This current will make normal control operation impossible.
Sensors: For Controls With Digital Displays
If the control has a digital display, leave the sensor wires connected and use the
control display to check the sensors. If the displayed temperatures do not appear
appropriate, disconnect the sensor wires from the control and use an ohmmeter to
check the sensor.
Many digital displays indicate short or open circuits by flashing the digits on display.
Again, an ohmmeter should be used on disconnected sensor wires.
NOTE
Whenever checking controls or sensors, also check to be sure the
collector with the sensor is properly grounded.
Freeze Snap Switches
In some systems, one or more freeze snap switches are installed in series or
parallel with the collector sensor. In others, they are part of a separate circuit.
Redundant sensors in different locations provide added protection against freezing.
Multiple freeze sensors are sometimes used in parallel or series to provide
redundant protection for collectors and exposed piping. Make sure they are in good
thermal contact with the piping or collector components, and that they are wired
correctly in series or parallel.
Be sure the freeze snap switches used with a control are made or recommended by
the control manufacturer. Some switches open on a temperature drop, and others
close on a temperature drop. The use of the wrong snap switch can destroy the
collectors by allowing them to freeze.
INSPECTION
90
3.1 INSPECTION PROCEDURES
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